Various approaches to automated or semi-automated three-dimensional object production or Rapid Prototyping & Manufacturing have become available in recent years. Each approach is characterized in that it typically proceeds by building up 3D objects from 3D computer data descriptive of the objects in an additive manner from a plurality of formed and adhered laminae. These laminae are sometimes called object cross-sections, layers of structure, object layers, layers of the object, or simply layers (if the context makes it clear that solidified structure of appropriate shape is being referred to). Each lamina represents a cross-section of the three-dimensional object. Typically laminae are formed and adhered to a stack of previously formed and adhered laminae. In some RP&M technologies, techniques have been proposed which deviate from a strict layer-by-layer build-up process wherein only a portion of an initial lamina is formed, and prior to the formation of remaining portion(s) of the initial lamina, at least one subsequent lamina is at least partially formed.
According to a first approach (i.e. stereolithography), a three-dimensional object is built up by applying successive layers of unsolidified, flowable material to a working surface. The layers are then selectively exposed to synergistic stimulation in desired patterns, causing the layers to selectively harden into object laminae which adhere to previously-formed object laminae. Layer formation and solidification, in some circumstances, may occur simultaneously or almost simultaneously. In this approach, material is applied to all portions of the working surface. The material is applied to areas which will not become part of an object lamina, and to areas which will become part of an object lamina. Some basic aspects of stereolithography (SL), are described in U.S. Pat. No. 4,575,330, to Hull. According to one embodiment of Stereolithography, the synergistic stimulation is radiation from a UV laser, and the material is a photopolymer. Another example of stereolithography is called Selective Laser Sintering (SLS), as described in U.S. Pat. No. 4,863,538, to Deckard, in which the synergistic stimulation is IR radiation from a CO.sub.2 laser and the material is a sinterable powder. A third example of stereollithography is called Three-Dimensional Printing (3DP) and Direct Shell Production Casting (DSPC), as described in U.S. Pat. Nos. 5,204,055 and 5,340,656, to Sachs, et al., in which the synergistic stimulation is a chemical binder, and the material is a powder consisting of particles which bind together upon selective application of the chemical binder.
According to a second such approach (Laminated Object Manufacturing--LOM), an object is formed by successively cutting object cross-sections, having desired shapes and sizes, out of sheets of material to form object laminae. Typically in practice, the sheets are stacked and adhered to previously cut sheets prior to their being cut, but cutting prior to stacking and adhesion is possible. Examples of this approach are described in U.S. Pat. No. 4,752,352, to Feygin; U.S. Pat. No. 5,015,312 to Kinzie; U.S. Pat. No. 5,192,559, to Hull, et al.; U.S. Pat. No. 5,524,232 to Burns; and EP Patent Publication No. 738583 to Morita, et al.
According to a third such approach, object laminae are formed by selectively depositing an at least partially unsolidified, flowable material onto a working surface in desired patterns in regions which will become part of an object laminae. After or during selective deposition, the selectively deposited material is solidified to form a subsequent object lamina which is adhered to the previously-formed and stacked object laminae. These steps are then repeated to successively build up the object lamina-by-lamina. This object formation technique may be generically called Selective Deposition Modeling (SDM). The main difference between this approach and the first approach is that the material is selectively deposited only in those areas which will become part of an object lamina. Examples of this approach are described in U.S. Pat. Nos. 5,121,329 and 5,340,433, to Crump; U.S. Pat. No. 5,260,009, to Penn; U.S. Pat. Nos. 4,665,492, 5,134,569, and 5,216,616, to Masters; U.S. Pat. No. 5,141,680, to Almquist, et. al; and U.S. patent application Ser. No. 08/535,772, to Leyden, et al.; Ser. No. 08/534,477, to Thayer, et al.; Ser. No. 08/722,335, to Leyden, et al.; and Ser. No. 08/722,326, to Earl, et al.
As noted above, stereolithography is a technique for stepwise or layer-by-layer build up of three-dimensional objects. In one embodiment of stereolithography, a three-dimensional object is formed in accordance with a corresponding object representation through a stepwise laminar buildup of cross-sections of the object at the working surface of a building material such as a polymerizable or solidifiable resin contained in a container. Each cross-sectional lamina is formed at the working surface of the building material by selectively transforming the material to a different physical form, such as a polymerized or otherwise solidified form, through exposure to a synergistic stimulation such as UV radiation. The cross-sectional laminae, as they form, are adhered to previously-formed laminae through the natural adhesive properties of the polymerizable resin as it solidifies. FIG. 1a illustrates a side view of a portion of an object 1 (the shaded area) formed by the layer-by-layer buildup of stereolithography. The layers forming the object are identified by numerals 1a-1i, respectively. Also shown is an object representation 2 according to which the object 1 is formed. The object representation 2 may be a data representation of the surface of the object, originating from a computer-aided design ("CAD") system or otherwise constructed by or put into a computer.
Because the layers forming the object have finite thicknesses, the object formed by a stereolithography apparatus ("SLA") or other layer-by-layer formation technique may have stair-step surface discontinuities, such as the areas identified by numerals 3a-3g in FIG. 1a. These surface discontinuities represent the deviations between the formed object 1 and the object representation 2. Such discontinuities generally lower the surface resolution of an object formed by an SLA. The magnitude of the surface discontinuities is partially determined by the thicknesses of the layers forming the object. Thicker layers typically lead to more significant surface discontinuities. FIG. 1b shows an object 1' formed according to the same object representation 2 as in FIG. 1a, but with thinner layers as identified by numerals 1a'-1r'. For object 1', the surface discontinuities, identified by numerals 3a'-3n', are less significant than the discontinuities for object 1 in FIG. 1a. In other words, the surface of the formed object 1' more closely matches the object representation 2. In addition to layer thickness, the magnitude of a surface discontinuity also depends on the geometry of the object surface. As shown in FIG. 1a, surface discontinuities are negligible at horizontal regions 2a and 2b, and vertical region 2c of the object representation, but are material at those surface regions that are neither completely horizontal nor completely vertical.
Several methods for improving object resolution have been proposed. One is to use building layers that are thin enough to adequately reduce surface discontinuities. This technique, however, is not generally feasible with the typical combinations of building material and synergistic stimulation available. The problems associated with this technique might include lack of structural strength during object formation, difficulties in controlling the precise layer thickness of the initially supplied unsolidified material, and longer object building time. Other proposed techniques for improving object resolution, as described in U.S. Pat. No. 5,184,307, involve post-processing steps such as sanding the formed part, or filling the surface discontinuities of the formed object with building material and subsequently curing it. These post-processing methods often involve manual processing. Manual processing can lead to inaccuracies in final object size and shape and can be time-consuming and/or expensive.
Methods for increasing object resolution by intermittent formation of fill layers inside the lamina-to-lamina surface discontinuities are disclosed in U.S. Pat. No. 5,209,878 (the '878 patent). One method of intermittent formation of fill layers is illustrated in FIG. 2, which shows two successive cross-sectional layers 4a and 4b of the object being formed (referred to as structural layers), and fill layers 6a-6c (depicted as hatched blocks). The fill layers 6a-6c have a thickness equal to a fraction of the thickness of the structural layer. Also shown in FIG. 2 is the object representation 5, against which the structural and the fill layers abut. It is seen from FIG. 2 that the object surface formed by structural layers 4a and 4b and the fill layers 6a-6c more closely matches the object representation 5 than an object surface formed by structural layers 4a and 4b alone. By using the fill layers, the structural layers can be formed at a greater thickness, yet the resulting object has reduced surface discontinuities equivalent to the discontinuities of an object formed with layers as thin as the fill layers.
The fill layers are formed intermittently with the adjacent structural layers, as opposed to being formed in a post-processing step after all the structural layers have been formed. For example, the layers may be formed in the order of 4a, 6a, 6b, 6c, 4b; or 4a, 4b, 6c, 6b, 6a. The latter sequence may require irradiating the building material through a previously formed layer 4b to form the fill layers 6c-6a. As the fill layers are formed, they are adhered to adjacent structural layers and/or previously formed fill layers. Other configurations for fill layers are described in the above referenced '878 patent.
The implementation of the fill-layer method is described in the '878 patent for an SLA using triangle data as an input object representation. Triangle data (typically provided in a form known as the STL format) is a form of input data for an SLA as manufactured by 3D Systems, Inc. of Valencia, Calif., in which the surface of the object to be formed is described by a plurality of tesselated triangles that collectively enclose the object. Triangle data may be generated, for example, by a CAD system. The three-dimensional object represented by the tesselated triangles may be "sliced" to obtain a set of cross-sectional data. The sliced data may describe the intersections of the object surface with a plurality of spaced parallel planes. Alternatively, the slice data may describe the object in a more complete and consistent manner such as in terms of an oversized, undersized, or other sized build style. The cross-sectional data may then be used by an SLA to control the laminae formation. Each cross-section of data corresponds to one lamina of the object, and the thickness of the laminae preferably corresponds to the distance between the successive parallel planes used for slicing. In practice the successive parallel planes may be thicker so as to allow adequate cohesion between layers or simply adequate cohesiveness within a single lamina. Methods for slicing a three-dimensional object represented by triangle data, including the primary features of commercial software programs SLICE and CSLICE, developed by 3D Systems, are described in U.S. Pat. Nos. 5,184,307; and 5,321,622, respectively. To implement the fill-layer methods for triangle input data, the object may be sliced at the thickness of the fill layers to obtain data for forming such fill layers. Alternatively, as explained in the '307 and '878 patents, slicing may occur at the spacing of the structural layers wherein fill layer data may be generated directly from sloping triangles (termed near-flat triangles in the '307 patent) that bridge the intermediate regions between successive structural layer contours. Techniques for deriving fill layer data or intermediate data from complete surface representations (e.g. STL files) of the object are described in the '878 patent in terms of preferred modifications to the SLICE or CSLICE programs.
Another technique for reducing lamina-to-lamina surface discontinuities is called meniscus smoothing. Meniscus smoothing is described in the '878 patent, and is illustrated in FIGS. 3a-3d. FIG. 3a shows a portion of an object formed by structural layers 8a-8d, and meniscus regions 9b-9d (depicted as hatched regions) that are formed adjacent the structural layers. These additional meniscus regions at least partially fill the step-like discontinuities formed by the structural layers.
The formation of a meniscus region is illustrated in FIGS. 3b and 3c. FIG. 3b illustrates formation of meniscus 9 in an up-facing discontinuity created by an up-facing surface area 11a of layers 8a and an end area 10b of layer 8b. The first step is to form layer 8a by exposing the building material, such as polymerizable resin, to a synergetic simulation such as a laser beam while the working surface of the building material is at level L1. Next, layer 8b is formed after the working surface has been relatively moved to level L2. Next, the level of the working surface is relatively moved to level L3, which is depicted as being the same as level L1. Alternatively level L3 may be different from level L1. For example, it could be lower relative to the object (i.e. layer 8b could be raised higher out of the building material). Because surface tension of the liquid material in combination with the surface energy of the solidified material, as the material recedes from above the up-facing area 11a adjacent the end area 10b, a meniscus 9 may be formed by the building material which remains in the discontinuity as shown. The next step is to expose all or a portion of the meniscus to the synergistic stimulation, thereby transforming it to the solid form. The result is a smoothed-over surface 9 that will more closely match the object surface 7. Similarly, FIG. 3c illustrates formation of meniscus 9 in a down-facing discontinuity created by a down-facing area 11b of layer 8b and an end area 10a of layer 8a. The first step is to form layer 8a while the working surface is at level L1. Next, relative to the object, the working surface is moved to level L2, and layer 8b is formed. Next, the working surface is relatively moved down to level L3, and as shown, a meniscus 9 formed by the working material may remain in the discontinuity. Finally, all or a portion of the meniscus is exposed and transformed by directing the synergistic stimulation through the already-formed layer 8b. Since the exact shape and size of the meniscus may not be known, an exposure may be given which will expose as much of the meniscus as possible without risking the passing of significant radiation through to the building material at working surface L3 which is to remain unexposed. Again, the result is a smoothed-over surface 9 that will more closely match the designed object surface 7. The meniscus smoothing process may be repeated to form multiple meniscuses on top of each other in the same discontinuity. For example, as shown in FIG. 3d, meniscus 9' may be formed over meniscus 9, and meniscus 9" over meniscus 9'.
In addition, meniscus smoothing may be used in combination with the fill-layer method, for example, by performing meniscus smoothing for each fill layer to reduce the remaining surface discontinuities after fill-layer formation. These techniques are also described in the '878 patent. Alternatively, meniscus smoothing could be performed for up-facing discontinuities while down-facing discontinuities may be cured with either fill layers, or with an exposure that continuously varies from the deepest part of the discontinuity to the shallowest part. If the object as formed, is going to be used as a master for tooling and if the tool only requires up-facing surface smoothness and accuracy, it may be acceptable to leave the down-facing regions unsmoothed.
As with fill layers, it may be inappropriate to meniscus smooth all surface lamina-to-lamina discontinuities as some discontinuities could be an object design feature. For example, as illustrated in FIG. 1a, the object surface 2 may have a "step" feature, such as that identified by numeral 2b, where an up-facing area of layer 1c coincides with the true object surface. In this situation, smoothing should not be performed over the up-facing surface area 2b, or at least not over the entire area 2b. A need exists in the field, when working with initially supplied contour data, for a technique to reliably estimate which discontinuities should be filled and which should be left alone.
Up to this point, accurate implementation of the fill-layer and meniscus smoothing techniques described in the '878 patent has required that data descriptive of intermediate regions be supplied so that it was known which cross-sectional discontinuities were originally part of the object design and which resulted strictly from the layer-by-layer formation process. Where the initial object data indicated that an intermediate region was sloped (i.e. intended to be smooth not discontinuous) appropriate fill layer data could be generated. Accurate implementation of fill layers could not be performed when a type of input data was initially supplied that did not include information about intermediate regions. An example of such initially supplied data is contour data which may be supplied in the standard Stereolithographic Layer Contour format (SLC format). The contour data format describes a three-dimensional object by a set of cross-sectional contours representing the intersection of the object surface with a plurality of spaced parallel planes. The parallel planes are preferably equally spaced and the planes may or may not be located with the same spacing as required for forming the three-dimensional object. Contour data is typically produced, for example, during medical diagnostic processes by computer tomography (CT) techniques based on data generated by a scanning device such as X-ray, magnetic resonance imaging (MRI), magnetic imaging angiography (MRA), positron emission (PET), or ultrasonic radiation. Such data can be used to form implantable prostheses by stereolithography, or other RP&M system, as disclosed in PCT Publication No. WO 95/07509.
However, since contour data is not descriptive of the object surface between the contours (e.g. intermediate regions), it is uncertain whether or not any surface discontinuity between cross-sections should be smoothed with fill layers. As a result, the fill-layer technique and the meniscus smoothing technique described in the '878 patent cannot be directly applied to contour data. Meanwhile, the problem of lamina-to-lamina surface discontinuity may be especially severe for an object formed from some types of contour input data. For example, this may occur because the contours obtained from medical applications are typically relatively widely spaced, so that fewer tomographic layers are needed and the radiation dosage received by the patient during scanning is minimized. For example, layer data may be obtained at 1.0-3.0 mm intervals. This is much greater than the typical layer thickness of the structural layers used in a rapid prototyping and manufacturing system (e.g. structural layer thickness utilized in an SLA is typically 4 to 6 mils (approximately 0.1 to 0.15 mm) though it may extend upward to or beyond 20 mils (approximately 0.5 mm) in some circumstances. Thus, an object formed directly from contour data initially supplied at a 1.0 to 3.0 mm thickness will typically be formed with stair steps (i.e. lamina-to-lamina surface discontinuities) of the same height as the spacing of the initially supplied data as several identical laminae are formed prior to the data dictating a change in cross-sectional shape.
One way to utilize contour data to form an object, with reduced surface discontinuities, is to transform the contour data into triangle data, which may then be sliced at a desired thickness to form the object. An application of this method in anatomical modeling is described in PCT Publication No. WO 95/07509. Once the contour data is transformed into the triangle format, one of the fill-layer techniques or meniscus smoothing techniques may be implemented in the same manner as described in the '878 patent to reduce surface discontinuities. This approach, however, still fails to address the issue of intelligently and reliably estimating how a gap between cross-sections should be bridged.
Using initially supplied contour data also created uncertainty as to the accurate sizing of individual cross-sections. It was uncertain whether or not each cross-section or portion of each cross-section was representing an oversized, undersized, or other sized structure. To over-, under-, right-, or otherwise size the object cross-sections requires a knowledge of how the actual object is intended to transition from cross-section to cross-section or lamina-to-lamina. The sizing of objects is discussed in detail in the '307, '622 patents and in U.S. patent application Ser. No. 08/428,951, to Smalley, et al. As noted above, when using contour data as the initial input data, it has not been possible to make a reasonable determination as to where to place fill layers, solidify meniscus regions, or to uniformly size the laminae. It was of course possible to assume that fill layers, or meniscus regions should have been used to fill every discontinuity between cross-sections or to assume that none of the lamina-lamina surface discontinuities should be filled. Alternatively, it may have been assumed that only smaller discontinuities than a preset maximum would be used. Therefore, a need exists for a more reliable method of and apparatus for deriving intermediate region data (i.e. a method of and apparatus for interpolating between initially supplied contour data to derive intermediate data descriptive of at least portions of intermediate contour levels).
All the U.S. patents, U.S. applications, and PCT and EP publications referred to herein above, in this section of the application, are hereby incorporated by reference as if set forth in full herein.